In the production of lead-acid storage batteries of the art, the positive plates are matured and dried in batches or continuously in so-called maturing and drying chambers after pasting the grids with the positive active material. From the main ingredients in the form of lead oxide, water, and lead sulfate, tribasic (3PbO·PbSO4) and/or tetrabasic (3PbO·PbSO4) lead sulfates are formed by maturation. Predominantly, the plates are laid in stacks on pallets without being separated. They are less often placed upright on pallets without separation, or hung with external lugs loosely in frames as occurs in the special case of double grids.
For the maturing to a tribasic lead sulfate with crystal sizes <10 μm, it is common practice to mature and subsequently dry the plates at about 55° C. over a period of time from 12 to 24 hours. Depending on the type of oxide that is used, as well as the desired residual moisture, drying will take as long as a few days.
As a function of the chemical and physical conditions, a phase transition from the formation of tribasic to the formation of tetrabasic lead sulfate occurs in the temperature range from 60° to 70° C. For maturing to a tetrabasic lead sulfate, it is common practice to mature the plates in water vapor for a few hours at a temperature of normally>80° C., and to dry them thereafter in the same way as the plates that are matured to tribasic form. A great disadvantage in the case of such a maturing by the action of water vapor is the development of coarse-crystalline tetrabasic lead sulfate crystals. In this instance, there may be crystal sizes>50μm.
During the subsequent formation, the matured active material of the positive plates is electrochemically converted to lead dioxide. Along with the increasing crystal size, the conversion of the basic lead sulfates becomes costlier and lengthier. The required amount of electric energy for converting a coarse-crystalline structure is by more than 25% higher than that of a fine-crystalline structure. In this instance, a “fine-crystalline” structure is understood a material of a crystal size <10 μm. In the case of a coarse-crystalline structure, crystals >30 μm are present. For a complete formation, it is moreover necessary to include residence times. By applying larger amounts of energy and the necessity of residence times that are to be included, the formation of coarse-crystalline, tetrabasic lead sulfate will as a rule take a substantially longer time.
The maturing into tetrabasic lead sulfates is advantageous in the case of lead-acid storage batteries with antimony-free alloys for the positive grids. Lead-acid storage batteries with antimony-free alloys for the positive grids and positive active materials that are matured into tetrabasic form, exhibit a stable capacity during a cyclical load, and they have a clearly longer service life. Lead-acid storage batteries with antimony-containing alloys of the positive grids are increasingly replaced with antimony-free grids, since these lead-acid storage batteries have moreover a longer shelf life as well as a visibly smaller water consumption.
For this reason, there exists a great interest in methods and possibilities of maturing positive plates into a small-crystalline tetrabasic lead sulfate. In the art, two methods stand out:
According to a common production practice, the plates are first matured to become tribasic and dried to advantageously less than 0.5 wt. % residual moisture. Subsequently, a water vapor treatment occurs over several hours at temperatures of normally >80° C. During this phase, the tribasic lead sulfate is converted into tetrabasic lead sulfate. In this process the crystal size remains almost unchanged, provided the moisture does not exceed about 2 wt % during the water vapor treatment of the plates. In the case of too moist plates, a growth to coarse-crystalline tetrabasic lead sulfate will occur. In the case of a properly conducted process, plates with small-crystalline tetrabasic lead sulfate are present after a subsequent, renewed drying process. A great disadvantage of this method lies in the long process time. Moreover, the paste-grid bonding is inferior to the positive plates that are matured in water vapor directly to coarse-crystalline tetrabasic lead sulfate. The size of the crystals of the tetrabasic lead sulfate cannot be controlled, and is <3 μm. In the case of a cyclic total discharge of wet lead acid storage batteries, this may lead to irreparable damage of the positive electrodes and, thus, to a shortening of the usable life of the lead-acid storage batteries.
In a second known method, previously finely ground tetrabasic lead sulfate is added to the active positive material during the production process. The maturation occurs in the same manner as in the case of the described maturation into coarse-crystalline tetrabasic lead sulfate by the action of water vapor and preferably at temperatures above 80° C. The added, finely ground tetrabasic lead sulfate crystals having a diameter <1 μm act as nucleators and allow individual plates a controlled crystal growth to a fine-crystalline tetrabasic crystal structure. Preferably, this method is carried out continuously.
The disadvantage of this method is the necessity of separating the plates, for example, by hanging double plates in spaced relationship or by a climatic diaphragm between individual plates. It is currently common practice in the production of plates for lead-acid storage batteries to stack them, after pasting, without spacers, and to mature them while being stacked. The necessity of singling the plates thus represents a considerable extra expenditure. As a result, it is not possible to use present systems and techniques for the plate production without new, additional devices or considerable modifications. The separation of the plates by spaces therebetween or climatic diaphragms leads to the need of more floor space, thereby reducing the capacity of plates in existing maturing and drying chambers to a great extent.